Many commercially available fluid pumps are designed to be driven by an electric motor. For pumps such as axial piston pumps, when the input shaft is driven at a constant velocity, the pump typically exerts a cyclically varying torque load on the input shaft. That is, with each rotation of the input shaft, the torque required to keep the shaft moving at the constant velocity cyclically rises and falls.
An electric motor for driving the pump is required to provide, at its minimum torque output, a torque greater than the maximum torque required by the pump. This ensures that the electric motor will always have sufficient torque output to drive the pump at the desired power level. Failure to ensure that the motor torque is sufficiently large results in stalling of the motor and pump unit during high hydraulic load conditions at both high and low rotational velocities of the motor. Stalling of the motor at high rotational velocities may sometimes be overcome using a high inertia rotor motor. However such motors are generally heavier and less efficient than low inertia motors and system designers typically prefer low inertia motors where system weight and efficiency are important.